The Basic Igneous Rocks of Eastern Otago and Their Tectonic Environment.

By W. N. Benson.
With Chemical Analyses by F. T. Seelye.

Part I–The General Tectonic Environment.

Introduction and Acknowledgments.

The detailed study of the Dunedin region which the writer is bringing to a close reveals the presence of a very long and varied sequence of Late Tertiary, more or less alkaline, igneous rocks covering 110 square miles out of a total of 250 square miles mapped in detail within a radius of 20 miles from the site of the earliest eruptions near Port Chalmers, eight miles north-east of Dunedin. The great diversity of these igneous rocks has been made clear by P. Marshall (1904, 1906, 1912, 1914, etc.) in a series of valuable papers. The writer's subsequent studies have indicated that their eruptions took place during a prolonged period of crustal instability and were immediately preceded, accompanied and succeeded by crust-movements, the climax of which occurred after cessation of the main eruptive activity. This tectonically and petrologically complex area will hereinafter be termed the Central Region of the Eastern Otago Pliocene petrographic province. In order to view it in proper perspective it is necessary to consider also the tectonically and petrologically less diversified peripheral regions characterised by approximately coeval igneous rocks lying between twenty and ninety miles from Port Chalmers, in which, within an area of about sixteen hundred square miles, over a hundred small masses of basic volcanic rock have been mapped, usually dykes, plugs, or residuals (rarely more than a few hundred acres in extent) of more extensive but relatively thin flows.

The peripheral regions south of the Shag River and more than five miles from the coast have a foundation similar to that in the Central Region, the quartz-albite-muscovite-chlorite-epidote schists of Otago, which merge southwestwards into less highly altered phyllonites and greywackes. (Cf. Turner, 1940.) The overlying pre-volcanic, usually quartzose sediments, are in general much thinner and are locally lacking. In the north and north-east, beyond the Shag River, the basement formations are greywackes and argillites, on which in the higher regions a similar sedimentary cover beneath the lavas is thin or lacking, but in the lower regions a considerable thickness, many hundreds of feet, of chiefly marine Upper Cretaceous to Lower Miocene sediments and mid-Tertiary basic volcanic and intrusive rocks occur, the petrographical nature of which has received some attention from Hutton (1889), Thomson (1906), Uttley (1918) and Marshall (1925).

The petrographical reconnaissance recorded in later parts of these papers has been aided by the studies of these authors, and by the work of Haast (1872, 1877), McKay (1887), Park (1918), Brown

(1938) and Marwick (priv. com.) in the regions between the Waitaki and the Shag Valleys; by that of Paterson (1941) and Service (1934) in the Lower Shag Valley and the adjacent Goodwood District, by collections made by Drs. Hutton and Turner in the Upper Shag Valley, by Dr. Turner's descriptions of the material collected by Williamson (1939) in the upper portion of the Taieri watershed, by that of Andrew (1906), Marshall (1912, 1918) and Ongley (1939) in the regions between Mosgiel and Balclutha. The writer is especially indebted to the Director of the Geological Survey for the opportunity of examining the material collected by Brown, Ongley and Williamson, and for permission to use certain data concerning the distribution of igneous rocks on the Kakanui Range when drawing Plate 36 herewith.

To these collections have been added specimens obtained by the writer, chiefly from the middle portion of the Taieri watershed, of which McKay's (1894) map was found to be inadequate, and is supplemented by that here offered, which, however, is still incomplete. In the petrographical study much help has been derived from Dr. F. J. Turner's optical determinations of various mineral-compositions by means of the universal stage, and by the series of excellent chemical analyses made by F. T. Seelye, A.I.C., through the courtesy of the Director of the Geological Survey and the Dominion Analyst.

Regional Tectonics.

(a) The Late Tertiary (Late Miocene?) Peneplain and Its Dislocation.

Cotton (1917, 1922) showed that the broad features of the topography of Otago resulted from the erosional modification of the surface of a series of usually relatively soft and locally lava-capped Cretaceous and Tertiary sediments, strongly dislocated in Late- or Post-Tertiary times, and resting on a peneplain cut for the most part during early Cretaceous times out of a basement series of schists, or (north of the Shag Valley), of greywackes and argillites. By the erosion in Late- and Post-Tertiary times most of the strata covering the Cretaceous peneplain were removed from the more elevated regions exposing the old land-surface, which became trenched by relatively youthful gorges, while the depressed regions have become covered by varying thicknesses of alluvium. The writer (Benson, 1935) pointed out that, in addition to Cretaceous peneplanation and very Late- or Post-Tertiary deformation and erosion, there occurred a significant Mid-Tertiary deformation followed by a peneplanation which not only stripped the cover from the more elevated portions of the Cretaceous peneplain, but in places reduced both such re-exposed areas and the surrounding region on which covering strata remained to a peneplain formed probably near the close of Miocene times. Subsequently, and in the main before any further significant deformation occurred, there were widespread outpourings of Late Tertiary (Pliocene?) volcanic rocks. The clearest indication of the position of the now deformed Miocene peneplain in any region is afforded by the base of the lava-flows therein, whether they rest on the stripped or re-planed surface of the basement-formations, or on the residual

Cretaceous-Lower Tertiary covering strata. “All recognised resurrected surfaces carrying residual buttes of covering beds or dipping beneath them are to be regarded as parts of the older peneplain, and where such remnants of the covering strata are thin and the basalts, if present, lie but little above the surface of the schist (or greywacke) the Cretaceous and Miocene peneplains are almost coincident…

Figure 1. Approximate contours at 200 ft. intervals on the deformed Miocene peneplain in the Dunedin District, showing in greater detail for this complex area the information charted on Plate 1, due allowance being made for the varying thicknesses of the interdigitating lava-flows which cover the peneplain where it has been depressed below sea-level.

The plateau, broken by occasional faults, which extends for scores of miles westward from Dunedin… probably incorporates parts of the two peneplains.” But in the absence of any remnant of the covering strata or lava the position of the Miocene peneplain is not demonstrable. Thus “it remains in doubt to which peneplanation must be ascribed the cutting of the extensive nearly level tops of the more conspicuous of the block-mountains.” (Cotton, 1938.) The writer must here express appreciation both for the generous acceptance of the “two-peneplain hypothesis” (which was, indeed, fore-shadowed in verbal suggestions by Professors Cotton and Douglas Johnson), and for the lucid statement of its essential features in the sentences quoted above. They explain adequately the principles upon which have been drafted the contour lines on the deformed Miocene peneplain shown in Plate 36. In part of the Central Region, illustrated in detail in Fig. 1, the volcanic rocks form a thick complex of interdigitating flows, much of which has been folded down beneath sea-level, so that the position of the base of this complex (the Miocene peneplain), can here be inferred by stratigraphic methods only, making allowance for the varying thickness and extent of the several flows. Quantitative uncertainty as to the extent of deformation of the Miocene peneplain is greatest where, as near Dunedin itself, allowance has to be made for the effect of faulting occurring during and after the period of volcanic activity. A qualitative accuracy is all that can be hoped for the contour-lines drawn below sea-level, but the fact that the Post-Tertiary dislocation reaches its greatest intensity in the central region seems to be clearly established. Again, since the very thick and dislocated Cretaceous (Kaitangatan) sediments in the ridge separating the Tuakitoto-Tokomairiro-Waihola-Taieri depression from the sea have been deeply eroded, so that only the feeding plugs of the volcanic rocks are exposed, all traces of the Miocene peneplain have here disappeared, and the anticlinal contour-lines sketched in this area have again only a qualitative significance. The position of the north-eastward-sloping Miocene peneplain on the Kakanui Range is indicated for that portion of it, wherein the distribution of the volcanic rocks petrographically determined to belong to the Late Tertiary series indicates the approximate coincidence of the Cretaceous and Miocene peneplains. Beyond the area occupied by residuals of these lavas in the north-eastern portion of the region mapped in Plate 36, the Cretaceous peneplain is overlain by a varying but considerable thickness of dislocated covering strata including Older or Mid-Tertiary volcanic rocks, the more or less deeply eroded surface of which affords no definitely residual areas from which the position of the Late Miocene peneplain may be determined, though an approximation thereto has been deduced from geomorphological considerations, with due allowance for differential sculpturing of very diveŕse formations as a result of the Post-Pliocene erosion of that warped surface. Because of this uncertainty, however, the discussion of the Late or Post-Tertiary dislocation is concerned chiefly with the regions south-west of the Kakanui Range. Haast's (1877) conception of the distribution and mode of occurrence of the Older

Tertiary dolerites, etc., near Moeraki is illustrated in Plate 36 and Fig. 2, Section A, but will need modification according to the observations made during the recent survey of that area.

Figure 2. Eight Sections drawn across the Eastern Otago region mapped in Plate 36. The numbers correspond with the locality-numbers shown on that Plate. Note that the mode of occurrence of the igneous rocks at Moeraki (shown by the shaded bands at the eastern end of Section A) illustrates the conception of their flow-nature figured by Haast (1877), which will be modified as a result of observations made during the preparation of the forthcoming Geological Survey Bulletin on the Moeraki Subdivision (fide D. A. Brown, priv. com.).

(b) Faults and Folds following a N.E.–S.W. Direction.

Two directions of faulting and folding are obvious, not only in the Central Dunedin Region, but throughout Eastern Otago. The dominant N.E.–S.W. trend (including all trend-lines within 25° of the exactly specified direction) is parallel to the general strike of the Late Tertiary folding, * the subordinate N.W.–S.E. trend (similarly generalised) follows the direction of the Late Mesozoic (Jurassic-Cretaceous) folding, and approximately the lineation of the schists (cf. Turner, 1940).

[Footnote] * Also the fault-lines and fold-axes, which in the vicinity of Dunedin trend N.70°E.—S.70°W.

Though much of the region mapped has been appropriately discussed as being characterised by block-mountains and fault-depressions (Cotton, 1917), it may, by an extension of Stille's (1922) concept of fault-folding (“Bruchfaltung”) of the Saxonian type, be considered as constituting a series of broad but broken asymmetric folds of Late or Post-Tertiary age, in which many of the steeper limbs have been more or less replaced by faults. Usually the western limbs of the anticlines have the gentler slopes and provide the stripped and partially dissected back-slopes of broken anticlines or fault-blocks, the eastern and steeper limbs passing into fault-scarps, which may often be traced downward into eastward dipping remnants of the limbs of the anticlines which, bent synclinally, pass into the gently sloping western limbs of the next succeeding anticline. When, however, the depressions have been deeply filled with alluvium, the distinction between an asymmetric, broken syncline and a fault-angle depression may not be possible. The transition between the fault-block structure and an obviously anticlinal structure is well exemplified by the Rock and Pillar Range, the northern portion of which, as Cotton (1917) has shown, is, for the most part, bounded by fault-scarps 1000–3000 feet high, facing the Strath Taieri depression, but terminates in a northward-plunging, broken anticlinal fold with gentle curvature, and capped by a thin sheet of Late Tertiary basalt. In so far as this lava-covered area may be considered a faulted, basalt-capped block-mountain, it accords with Davis' (1930, p. 299) definition of a louderback. The transition towards an anticlinal structure seems to illustrate Bucher's (1933, p. 322) comment that in regions which have been affected by crust-movements resembling in some respects those of the Saxonian type (cf. Benson, 1930, p. 13), “the major upthrown blocks are not merely passive blocks of a foundered crust, but are elongated in the direction of the axis of folding and partake somewhat of the nature of anticlines. Their borders are here and there overthrust on to the sediments of the depressed blocks. The sediments on the upthrown blocks show little or no folding. In the down thrown blocks the intensity of the folding grows with the thickness of the sediments. A large number of unconformities and their local presence or absence from place to place are characteristic.” Williamson's (1939) descriptions of the long sequence of sediments in the Maniototo depression, and their occasional interformational unconformities and overthrusting seem to support the comparison which has been drawn.

The asymmetric Silver Peak, Flagstaff-Swampy Hill and Mount Cargill-Mihiwaka-North Head ridges with the intervening Silver-stream-South Waikouaiti, and Leith-Waitati valleys in their faulted synclines (see Fig. 1), exemplify also the general south-eastward slope of the steeper limbs of the folds. The anticlinal ridge constituting the Otago Peninsula is nearly symmetrical in its eastern end, but its western end has its steeper slope directed away from the Pacific Ocean and probably largely replaced by a fault (Benson, 1940, Fig. 1, Section F.) This last is also true for the anticlinal ridge between the sea and the Taieri Plain-Waihola-Tokomairiro-Tuakitoto depression. In the faulted synclinal Maniototo-Serpentine Flat depression occupied by the Upper Taieri River, the steeper and more

faulted limb faces oceanward at the north-eastern end, and westward at the south-western end. The same is generally true in regard to the Otago Harbour syncline. It is to be remarked also that some of the faults, notably that on the north-western side of the Leith Valley (Benson, 1940, Fig. 1, Section F) are reverse faults, indicating movement under compressive forces. Hence a general underthrusting force, directed north-westwards from the ocean, may have dominated the Post-Tertiary compressive dislocations which trend in a N.E.-S.W. direction, though it has not had uniform and probably not continuous expression throughout Eastern Otago.

It seems here desirable to call attention to certain implications which may follow from the comparison suggested between the crust-movements in Central and Eastern Otago and those of the Saxonian type which affects areas in which relatively undisturbed normal* sediments rest on more rigid but formerly highly folded or crystalline foundations. Stille (1910, 1924, pp. 252–256) held that they result from periodic lateral compression (“episodische seitliche Zusam-menschub”), and that the faulting is a secondary effect of the anticlinal uplift. Willis (1927, p. 27), referring to the structure of the folded and faulted Tertiary rocks of the Coast Ranges of California (which Bucher compares with Saxonian tectonics), expressed a similar view. “There have been periods of deformation separated by periods of quiescence. During the latter, the relief of the surface of the fault mosaic was reduced by erosion of the highs, and sedimentation in the lows. It is possible that relaxation of the horizontal pressure may have caused gravitative subsidence of the previously compressed and elevated ranges, permitting widespread deposition over the basement of the fault mosaic.” Bucher (1933, p. 319) held that the term “relaxation” is inadequate, and cites Clark's (1930, p. 794) statement. “It seems probable that many of the primary faults in the Coast Ranges were normal and tensional faults during much of Cretaceous and Tertiary times. Most of them, at the time of the Coast Range revolution (Late Pliocene to Early Pleistocene), became compressional reverse faults, some of which developed into great thrusts. Some of those that were compressed in early Pleistocene time appear to be normal now. This alternation from normal to compressional, and then possibly a reversal to normal, brought about some very marked reversals of movement of the blocks along those lines.” Rubey (1934), however, reviewing Bucher's discussion, remarks that “a series of tilted fault-blocks do not necessarily prove regional tension.… Local tension may be the perfectly normal accompaniment of a more regional compression. Applying his view to the type European areas, Bucher agrees with Schuh (1922) and others that compression alone cannot break up a portion of the earth's crust many thousands of square miles in area into a mosaic of fault-blocks typically bounded by normal faults, especially where they are adjacent to regions in which fracturing rather than folding was the dominant effect, and concludes that their genesis took place under conditions radically different from those which result in folding and

[Footnote] * The effects of Saxonian movements in sedimentary sequences containing thick beds of rock-salt are, of course, without analogy in our area.

overthrusting. The structure of the Saxonian region, he holds, was the product of an alternation of epochs of tangential tension with those of tangential compression. There are, he thinks, all gradations between tensionally-faulted, unfolded and relatively immobile belts, tensionally-faulted and moderately folded belts of greater mobility, and very mobile belts in which folding was the dominant form of deformation. While it would be inappropriate to apply such conceptions directly to the by no means typically Saxonian structure of the Otago region, it seems probable that, in so far as comparison may be drawn, the transition between the relatively high-standing, strongly faulted, and slightly folded western or Central Otago portion of the area herein discussed on the surface of which the covering of Cretaceous and Tertiary sediments is thin or lacking, the positive elements in Schuchert's (1910) and Willis's (1907) sense, and the more negative depressed coastal portion with its thick covering of somewhat folded Cretaceous and Tertiary sediments, might be considered to exemplify such a fraction of the above-defined range of tectonic conditions as might be comprised within its middle term. An alternation of compressive and tensional tectonic conditions such as Bucher's view would imply if, notwithstanding Rubey's comment, it is applicable to Eastern Otago, would be of special interest in connection with the character and period of eruptions of the igneous rocks to be discussed in the second part of this paper.

(c) Faults and Folds following a N.W.–S.E. Trend.

Dislocations with a north-westerly trend are most strongly marked in the Shag Valley, the steep north-eastern side of which has been broken by a group of powerful faults following an ancient zone of fracture (see Cox, 1883). Cotton (1922, pp. 140, 156, 159–160) has pointed out that this scarp has a complex origin, being in part a fold-scarp and in part a fault-scarp. Near Green Valley, Waihemo, some fourteen miles north-west of Palmerston, the first component seems very important, if not dominant. Near the mouth of the Shag River, Paterson (1941) recognised a third component, the formation of a fault-line scarp, in the production of this very noteworthy topographic feature. It is to be noted that the steeper limb of this broken Shag Valley syncline faces south-westwards, and that the adjacent anticlinal Kakanui Range has a gentle north-eastward slope, but these directions are more nearly perpendicular than parallel to that of the inferred sub-oceanic thrust. The slopes south-west of the Shag Valley are broken by the N.W.–S.E. fault-determined valleys of McCormick's Creek and the upper portion of the North Branch of the Waikouaiti River, the formation of which valleys may have been, in part, preceded by removal of a faulted sheet of covering strata. The Castle Hill fault extending W.N.W.-E.S.E. in the warp forming the northern margin of the Barewood Plateau, and deflecting into an eastward direction the course of the river leaving the Strath-Taieri depression, and, indeed, the very broadly synclinal depression of which the partially dissected floor is represented by the Barewood Plateau, may be considered a further expression of this group of tectonic movements, which, in the Dunedin district is most clearly

marked by the Portobello-Port Chalmers anticline which began to develop during an early phase of the Pliocene period of volcanic activity. Other examples might be cited.

(d) Repeated Movement along Faults and Folds.

It has long been recognised that the Shag Valley fault-zone is one of the most ancient and important in Otago (Cox, 1883). The fact that it has brought into apposition the Palaeozoic (?) Otago or Maniototo Schist on the south-west with the Early Mesozoic (?) grey-wackes, etc., on the north-east which were reduced to a common peneplain level by the earlier Cretaceous erosion, indicates movement with a southerly upthrow during the closing Jurassic orogeny. Paterson (1941) pointed out that the semi-talus character of the oldest Upper Cretaceous rocks near Shag Point indicates the existence of a rejuvenated fault-scarp on the south-eastern side of this fault-zone. Erosion of the scarp yielded what is almost a fanglomerate until the scarp was reduced, the fault-angle depression to the north-west of it was filled and the relief obliterated before the latest portion of the period, and the Upper Senonian-Danian marine sediments, sandstones and glauconitic mudstones, and the various older Tertiary sediments were deposited across the fault-zone. Renewed movements, this time with a northerly upthrow, occurred in Mid-Tertiary times, as is demonstrated by the structure of the ridges between the Shag Valley and the main road on either side of Green Valley. Here, erosion, occurring after this faulting had taken place, reduced to a common peneplain-level, the upthrown Mesozoic (?) greywacke on the northeastern side of the fault, and the Lower Oligocene Waitakian limestone * below which are greensands and coal-measures which rest on the basement Otago or Maniototo Schist on the south-western side of the fault. In Pliocene (?) times a flow of basalt flooded over this peneplain crossing the (by then) obliterated fault-trace to rest on limestone and greywacke alike.

Movement was again renewed in Late or Post-Pliocene times, when the Kakanui Mountains rose immediately north-east of this fault-zone, largely as a very sharply bent fold-scarp, broken by several more or less parallel N.W.–S.E. faults following the general fault-zone. In spite of the generally northward upthrow, the actual displacement along the member of the fault-zone beneath the basalt shows a southerly upthrow of 100–200 feet making a small fault angle depression, into which have been incised the middle portions of the courses of Green Valley and Happy Valley, while the faults extending along the higher portions of the flanks of the fold-scarp seem also to have here and there produced fault-angle depressions. (See Fig. 2, Section A.) The whole structure suggests the action of strong N.E.-S.W. compressive stress producing the asymmetric anticlinal fold which constitutes the Kakanui Range accompanied by small-scale reverse faulting on its steeper flank. Further details supplementing these preliminary notes on the evolution of the Shag Valley will doubtless be given in Brown's forthcoming Bulletin on the Moeraki Subdivision. The information so far presented supports

(a) In black. Approximate contours (200 ft. interval) on the dislocated surface of the Late Miocene peneplain in Eastern Otago. The position of this surface is determined by the base of the Pliocene lava-sheets where these occur, and by the interred extension of this surface between the lava-residuals in conjunction with topographic evidence of faulting, and due allowance for post-Miocene erosion of regions without any lava-covering. The height of the Miocene peneplain along the present coastline varies from about 600 feet above the sea near Moeraki to about 800 feet below the sea at Otago Heads. Except in the Central Region near Dunedin it is generally several hundred feet above the present coastline.
(b) In red. Distribution of Pliocene igneous rocks, with numbered localities whence specimens of such rocks have been obtained from outside the Dunedin Central Region for description in the later parts of these papers. The relation between the intensity of deformation of the Miocene peneplain and the extent and variety of the Pliocene igneous rocks developed is indicated.

with additional information the general conception of this very interesting topographic feature which was set forth in Cotton's lucid discusson (1922).

Repeated movements along faults and folds with a N.E.–S.W. trend are not so readily demonstrated, but a number of examples of faults which had a marked Pre-Pliocene movement may be cited. The first of these to be noted is that west of Goodwood, which was adequately described and illustrated by Service (1934). Another is the South Waikouaiti River-Silverstream fault immediately east of Silver Peak (Benson, 1940, Fig. 1, Section F). On the west side of the fault the basalt-covered Miocene peneplain is separated from the underlying schists by at most a few feet of Cretaceous sandstone, while immediately east of the fault more than a hundred feet of sediments intervene between the schist and the basalt. Pre-Pliocene faults may be traced into areas from which all traces of the covering strata have been removed, and the resulting fault-line scarp is at first sight indistinguishable from a fault-scarp.

The southern end of Service's (1934) above-mentioned Goodwood fault has this nature where it extends into the ridge west of the junction of the north and south branches of the Waikouaiti River, and the same is true of some of the N.W.–S.E. fault-lines. It is hoped to discuss in another paper various cases of this feature as it is developed in the Waikouaiti watershed.

(e) Stratigraphical Evidence of Repeated Crust-movements.

In addition to such topographic proof of repeated movements, stratigraphic evidence of intermittent dislocation is also available. It has been shown that over most of the moderately high areas in the region mapped, whether they have or have not residual patches of Late Tertiary basaltic rocks, the present summit-level of the schist or greywacke is but little removed from that of the Miocene peneplain. It follows that these were areas of feebly “positive” character in Willis's sense, which had been uplifted without warping to such a height that the Mid-Tertiary erosion stripped the cover of Late Cretaceous–Older Tertiary sediments from the basement formations under the Cretaceous peneplain, which, when thus bared, had not sufficient elevation or declivity to permit its further reduction to any notable extent, except in the case of the stripped Cretaceous monadnocks, if any, and the more strongly “positive” anticlinal or elevated fault-block salients. Thick masses of the Cretaceous and Tertiary covering strata would be preserved only in the “negative” areas which had been warped down beneath the Mid-Tertiary base-level. Such thick formations to-day exist only in the present lowlands, which occur in regions downwarped during the Late or Post-Tertiary dislocations, notably the coastal regions and the Taieri-Tokomairiro and Maniototo plains, and also the Shag Valley. Williamson's (1939) account of the intraformational unconformities seen along the margins of the thick series of Cretaceous and Early Tertiary sediments in the Maniototo depression indicates the prolonged Pre-Miocene instability of that region of intermittent Saxonian movements. The difference between the probable thickness of the sediments beneath the dolerite rocks east of Ranfurly

(Plate 36, Loc. 68) and that of the very thin development beneath the same doleritic flow nearer Waipiata (Ibid., Loc. 66, 70) may be considerable.

The sequence of Cretaceous to Mid-Tertiary sediments near Green Valley in the Shag Valley comprises at least one notable disconformity, * but among those sediments in the coastal districts between Oamaru and the southern portion of the Dunedin Central districts, the disconformities, though present, are less obvious (Brown, 1938). South-west of the Shag Valley the Upper Senonian fossiliferous marine beds are known only in the Brighton-Green Island district, but their absence elsewhere above the underlying coal-measure sandstone does not prove the presence of a disconformity, as its upper limit is not necessarily coeval throughout this extent. The occurrence of Danian, Boulder Hill-Wangaloan beds immediately above the Coal-measure sandstone in two areas only, has with more reason been held to indicate a break in the sequence, for which indeed evidence of disconformable deposition has been cited (Grange, 1921; Ongley, 1939). The latest Cretaceous * Abbotsford Mudstones following on these are, especially within the Dunedin Central Region, usually covered by the Green Island Loose Sandstone, but near the Waikouaiti River they are followed by the Upper Eocene Burnside Mudstones, indicating a local disconformity or area of non-deposition. The presence near Dunedin of a bed of greensand 55 feet thick between this Loose Sandstone and the Burnside Mudstone affords additional evidence of disconformity in this sequence of sediments. (Cf. Goldman, 1921.) A more profound break, indicated by palaeontological evidence of the almost complete absence of Oligocene formations, is the separation of the Lower Miocene † Caversham sandstone from the Burnside marl by only a few feet of greensand. Thus in this relatively open region of Upper Cretaceous to Lower Miocene sedimentation slow intermittent subsidence is the only movement demonstrated.

Conditions were different, however, in the narrow Taieri-Waihola-Tokomairiro-Tuakitoto depression extending south-west-ward from Dunedin and separated from the ocean by an anticlinal ridge. This may be inferred from the Sections in Fig. 2, Nos. E, F, G, and Fig. 3 based in part on the data supplied by Ongley's (1939) map. The chief distinction lies in the extensive development of the pre-Senonian (Mid- or Early Upper Cretaceous?) Kaitangatan beds along the eastern side of the depression. As in the case of the Shag Point or Horse Range Conglomerate (cf. Paterson, 1941), so here the beds in question were most probably accumulated in a fault-angle depression. They seem to have been formed immediately to the west of a rising fault-block, and the higher portions of the series may contain the largest rock fragments. This point is stressed by Ongley (1939, p. 49), who shows the inadequacy of the formerly held moraine-hypothesis of the origin of these deposits, the maximum thickness of which he estimates to be about 3000 feet. It is, however, possible, though not necessary, that their richly feldspathic

[Footnote] * Brown and Marwick, verb. com.

[Footnote] † The age determinations are those of Finlay and Marwick (1939).

character may have been influenced by a continuance of the abnormally cool condition indicated by the feldspar-rich Aptian to Upper Albian sediments of East-Central Australia (Woolnough and David, 1926; David, 1932). The depression in which they were accumulated was very narrow throughout most of its extent (see Fig. 1, E, F, G), but widened southwards where Kaitangatan beds are overlain with slight unconformity by the quartz-conglomerates

Figure 3. Sections across the south-western end of the Taieri-Waihola depression, in part an enlargement of portion of Section F of Figure 2.

of the Senonian (?) Taratu Series (Ongley, 1939, p. 52). In the region illustrated in Fig. 3 the few feet of sandstone representing the latter formation has so far overlapped the Kaitangatan beds that it rests directly on the schists on the western side of the depression, while strongly dislocated Kaitangatan beds several hundreds of feet thick rest on the schists a mile away on the eastern side of the depression. Evidently the crust-movements between the times of deposition of the Kaitangatan and Taratu beds were locally very vigorous. Their effect has been indicated by the unconformity between the representatives of these beds in the western flank of Saddle Hill at the northern end of the Taieri Plain depression and on the southern margin of the Dunedin Central Region. In the narrowest part of the depression illustrated by Fig. 3 A, the Taratu sandstone is overlain by Abbotsford Mudstone not hitherto recognised here, * but at Clarendon (Fig. 3 B), four miles to the south, this late Cretaceous formation is missing as is also the overlying Eocene Green Island Loose Sandstone and Burnside Mudstone, and the quartz grits, here only a few feet thick, are followed disconformably by the Milburn limestone which has been correlated with the early Miocene (?) Caversham sandstone (Marwick, in Ongley, 1939, p. 60). The transition occurs through a zone of loose glauconitic sandstone, best seen in the quarry nearly a mile east of Milburn. Moreover, within the limestone itself there seems to be evidence of an erosion-surface about seventy feet above its base (Andrew, 1906).

[Footnote] * The age of a group of Foraminifera occurring in a chert above this mudstone opposite Waihola is unfortunately not determinable.

A strong Upper Cretaceous dislocation between the times of formation of the Kaitangatan and Taratu beds.

(b)

The possibility of warping and local disconformity accounting for the absence of the marine Senonian and Danian beds beneath the Abbotsford Mudstone at Waihola.

(c)

A greater disconformity determining the absence of the Late-Cretaceous to Oligocene formations at Clarendon and Milburn.

(d)

Possibly a very subordinate movement indicated by the erosion surface within the Lower Miocene limestone.

The distribution of the Pliocene volcanic rocks in the western portion of the Dunedin Central District suggests that a slight syncline extended eastwards and north-eastwards of the Taieri depression during that period. It further appears that the present Taieri-Waihola-Tokomairiro depression, which is a faulted syncline resulting from post-Pliocene crust-movement, is parallel to and at most a few miles west of the Mid-Cretaceous fault-bounded depression. Once again the tectonic character of Eastern Otago seems remarkably persistent.

David, T. W. E., 1932. Explanatory Notes to accompany a New Geological Map of the Commonwealth of Australia, p. 84, The Commonwealth Council for Sci. and Ind. Res., Australian Medical Publishing Co., Sydney.